A man is about two square meters of skin. Assuming they lay down to photosynthesize most efficiently from perpendicular sun rays that would be one square meter, so 1400 watts. You would need an ~11% efficient conversion to meet the needs of a human. Plants currently top out at about 6.4% for highly efficient C4 plants like sugarcane. 11% is the maximum efficiency possible.
>The photosynthetic efficiency is dependent on the wavelength of the light absorbed. Photosynthetically active radiation (400–700 nm) constitutes only 45% of the actual daylight. Therefore the maximum theoretical efficiency of the photosynthesis process is approximately 11%.
Not only would you have to somehow replicate the extreme efficiency of sugarcane on animal skin (an organ with no support structures for photosynthesis), you’d have to double the efficiency.
> Our body’s demand for glucose is higher than photosynthesis can accommodate. Associate professor Lindsay Turnbull of the University of Oxford determined that if the surface area of an adult woman contained chlorophyll like a leaf, it would produce only 1% of the daily energy requirements for the person to survive. To live by photosynthesis alone, the woman would need a green body with the surface the size of a tennis court.